Fig. 1

The HBA biosynthesis pathway and related modification strategies. a HBA biosynthesis pathway. The introduced foreign genes are marked blue, and the whole pathway was divided into a precursor module and an HBA module. The two branching pathways are shown in the red box. b An RBS library and combinatorial constructs were used to screen the optimal relative expression levels for the four heterologous genes hemABCD to achieve the purpose of optimizing precursor supply. c Weakening the expression of the native hemE and cysG genes on the chromosome